HTML Validator - Firefox add-on, open source by Marc Gueury. The validation is done on your local machine inside Firefox and Mozilla. Error count of an HTML page is seen as an icon in the status bar when browsing. Can validate the HTML sent by the server or the HTML in the memory (after Ajax execution). Error details available when viewing the HTML source of the page. Based on Tidy and OpenSP (SGML Parser). Available in 17 languages and for Windows and other platforms.